Dry Bulb Temperature - Tdb

The temperature axis in the diagram is also know as the dry bulb temperature (Tdb). This parameter is popularly known as temperature when people read the thermometer.
The dry bulb temperature is called "dry bulb" because the measured temperature is not affected by any moisture in the air.

Units:

• Degrees Celsius(°C). Zero Kelvin equals to -273°C.
• Degrees Fahrenheit(°F) - Imperial units

Wet Bulb Temperature - Twb

The Wet Bulb Temperature (Twb) represents the air temperature when fully adiabatic saturated (100% Relative Humidity). To make it more understandable: it is the temperature of a wet object in an air flow.
When the air flow is not saturated, moisture of the wet object will evaporate (adiabatic process) in the air which results in a lower temperature of that object; the wet bulb temperature of the air.
When the air flow is saturated (100% RH), the temperature of the air flow is equal to the wet bulb temperature of the air.
Adiabatic humidification follows the wet-bulb tempeprature lines and is slightly different from following the enthalpy-lines.

Units:

• Degrees Celsius(°C) at 100% Relative Humidity
• Degrees Fahrenheit(°F) at 100% Relative Humidity - Imperial units

Dew Point Temperature - Tdew

The Dew Point Temperature (Tdew) is the temperature where water vapor starts to condensate from the air. At lower temperatures the air can not hold the amount of water.
Like the Wet Bulb Temperature the Dew point Temperature is also fully saturated (100% Relative Humidity).
The humidity ratio of a random given air condition is the same as its Dew Point Temperature.

Units:

• Degrees Celsius(°C) at 100% Relative Humidity
• Degrees Fahrenheit(°F) at 100% Relative Humidity - Imperial units

Humidity Ratio - X

Humidity ratio (X) is the actual amount of water vapour that is absorbed by the air, regardless of the temperature of the air. Because it is not affected by the temperature, the volume of the air is also inrelevant. Therefore this parameter is given as a weight ratio:

• grams per kilogram (g/kg)
• millipounds per pound (mlb/lb) - Imperial units

Relative Humidity - %RH

The Relative Humidity (RH) is the the ratio in a percentage (%) of the moisture content (humidity ratio) compared to the amount of moisture that would be present if the air were saturated at the same temperature.
Relative Humidity is often abbreviated to "RH".

• 100% RH means that the air is saturated.
• 0% RH means that no moisture is present in the air.
• 30% RH means that 30% of the possible moisture weight is absorbed.

Enthalpy - H

Enthalpy (H) is know as the total amount of energy the air mixture. This energy can be expressed in sensible energy (absorbed temperature) and latent energy (absorbed moisture).
This means that an air mixture with a lot of moisture can have the same Enthalpy as air with less moisture absorbed but with an higher temperature.
Units:

• kiloJoules per kilogram (kJ/kg)
• British thermal unit per pound (BTU/lb) - Imperial units

Important to know when converting Enthalpy from metric to imperial units is that 0 kJ/kg is related to 0°C and 0 BTU/lb is related to 0°F (other zero point)